Nearly instantaneous global communication is close to becoming a reality through recent technological advances in data communications equipment. Among the factors hindering the realization of this goal is a lack of worldwide standards for certain operating parameters of data communications equipment. One of these non-standard operating parameters is the range of levels of the received signals from public switched telephone networks and leased lines to which modems must respond. In the United States, modem receivers are normally designed to handle signals within the dynamic range of -48 dBm to -10 dBm. In contrast, modems used in various foreign countries are required to respond to received signal levels which vary over the broader range of -48 dBm to 0 dBm. Thus, data communication equipment used in global communications, including modem receivers, must possess a broader dynamic range than devices used only in the U.S.A. and be able to reliably receive and respond to signals within the -48 dBm to 0 dBm range.
It is well known that the received level signal for a modem communicating over the public switched telephone network will vary over a considerable range, even if the transmitted level at the transmitting modem remains constant. The level of the received signal is impacted by the condition of local lines, the routing of the call and a number of other variables which impacts signal level in the PSTN. Additionally, there is a dynamic component affecting the signal level including short term signal dropouts and the like which require dynamic amplification of the signal received from the line.
Modern modem designs are rapidly migrating toward high levels of system integration on integrated circuits, also referred to as chips. At the present time, a typical integrated circuit implementation of a modem includes an analog front end (AFE) chip, a digital signal processing (DSP) chip, and a microcontroller. Typically, the analog front end includes a port to receive an externally provided signal from the telephone line input to the modem in which the chip will be used. The first element is an anti-aliasing filter, followed by some form of automatic gain control. The present invention resides in an improvement to the automatic gain control circuit in such an analog front end. Following the automatic gain control, a switched capacitor filter is used, the output of which is provided to an analog-to-digital converter (ADC) circuit.
Due to the large number of switching transients encountered in the switched capacitor filter, the filter tends to be the noisiest element in the AFE chip. It establishes the base line signal to noise ratio which is acceptable in order to meet the minimum requirements for an acceptable signal to noise level at the output of the analog-to-digital converter.
For example, in a 9600 bps modem receiver, the minimum acceptable signal to noise ratio is approximately 30 dB to maintain a 10.sup.-5 bit error rate. The majority of noise in the analog front end of this modem receiver is generated by the switched capacitor band pass filter, which generates noise on the order of 500 microvolts or -63.80 dBm. Thus, the amplified incoming signal must be at least -33.8 dBm for acceptable performance, calculated as follows: ##EQU1## If the incoming signal is not at least at a level of -33.8 dBm, it must be amplified in order to maintain a minimum signal to noise ratio. As noted above, modem receivers used in the United States are designed to handle incoming signals with a dynamic range of -48 dBm to -10 dBm. Pursuant to the example above, if the incoming signal level was -48 dBm, only 14.2 dB of amplification would be required to boost the signal to -33.8 dBm. This would provide the acceptable minimum 30 dB signal to noise ratio. However, global communications require that modem receivers be equipped to handle incoming signals with a dyanmic range of -48 dBm to 0 dBm.
In prior art modem receivers, the level of the incoming signal is determined by a received level detector. This information is passed on to a controller and the level of the incoming signal is amplified as necessary to achieve an acceptable signal to noise ratio. In one example from the prior art, the present inventors designed a custom integrated circuit chip for the analog front end of a 9600 bps fast turnaround modem using CCITT Recommendation V.32 modulation. This prior art analog front end is more fully described by the present inventor. R. Halim and D. Shamlou, in the article entitled "An Analog Front End for High Speed Fast Turnaround Modems" which was presented at the IEEE CICC Conference in May 1989. In this prior art modem, the incoming signal level is detected by a received level detector in the AFE chip and the incoming signal is then amplified by one of the discrete gain levels of 0,6 or 12 dB as necessary to assure at least a minimum signal to noise ratio.
The received level detector comprises two parallel circuits, threshold detector-1 and threshold detector-2, each comprising an analog threshold comparator (ATC). In operation, each ATC compares the level of the incoming signal to a particular reference level. ATC-1 compares the incoming signal level to reference level-1 while ATC-2 compares the incoming signal level to reference level-2, which is 6 dB higher than reference level-1. After comparison, if the ATC determines that the incoming signal level has exceeded the assigned reference level, the ATC will output a logical 1 state. If the incoming signal level is determined to be below the reference level, the ATC will output a logical 0 state.
The output from the ATC is provided to a latching counter. The latching counter will latch to a logical 0 after a predetermined number of clock transitions with the signal from the ATC in a logical 0 state. The latching counter will latch to a logical 1 after a predetermined number of clock transitions with the signal from the ATC in a logical 1 state. The latched state of each threshold detector is then communicated to the controller which decides whether amplification of the incoming signal is necessary, and how much, if any, amplification is to be enabled according to the latched states as shown in Table 1 below.
TABLE 1 ______________________________________ TD-1 TD-2 Amplification ______________________________________ 0 0 12 dB 0 1 Invalid state 1 0 6 dB 1 1 0 dB ______________________________________
From Table 1, an output of a logical 0 from both threshold detectors indicates that the incoming signal level is below both reference levels. Accordingly, the incoming signal is amplified by 12 dB. An output of a logical 0 from threshold detector-1 and a logical 1 from threhold detector-2 is an invalid state because the second reference level is higher than the first. It would not be possible for the incoming signal level to fall below the first reference level yet exceed the second reference level. An output of a logical 1 from threshold detector-1 and a logical 0 from threshold detector-2 indicates that the incoming signal level has exceeded the first reference level, but did not exceed the second reference level. In other words, this output indicates that the incoming signal level is between reference level 1 and reference level 2. Accordingly, the incoming signal is amplified by 6 dB. An output of a logical 1 from both threshold detectors indicates that the incoming signal level has exceeded both reference levels. Accordingly, the incoming signal is not amplified.
The maximum amount of amplification provided by this AFE chip is 12 dB. This poses a limitation on efforts to broaden the dynamic range of the prior art modem for global communications. Broadening the dynamic range as described above would require that the modem receiver amplify the incoming signal level by more than 12 dB. To accommodate a broader dynamic range in accordance with the detection system of this prior art AFE chip, a third parallel circuit would have to be added to the received level detector, i.e. a third threshold detector. This would include adding another analog threshold comparator with another reference level source. Thus, pursuant to the previous example, adding a third threshold detector to the received level detector would allow for amplification of the incoming signal by 0 dB, 6 dB, 12 dB or 18 dB.
From a circuit design point of view, it is apparent that expansion of the dynamic range of the AGC from 12 to 18 dB, while maintaining 6 dB steps, could be accomplished in a straightforward manner by adding a thrid threshold detector of the type described above. This represents a straightforward extension of the technique adopted in the above described AFE chip to expanding its dynamic range. However, this is an undesirable solution because of the relatively large chip area occupied by the threshold detector circuitry and the complications this would add to the layout of the chip. As is well known to those skilled in the art, moving to a larger die size significantly increases the expense of a given chip. Fabrication is more difficult and yield rates drop as the die size increases. Therefore, the present inventors were presented with a situation in which there was a strong need to modify the above described AFE chip in a manner which would allow expansion of the AGC gain range from 12 to 18 dB without unduly complicating the chip design or taking up more chip real estate. In other words, there was a need to expand the dynamic range without increasing the number of threshold level detectors, which increase yields an apparent solution.
However, it will be clear that the apparent solution violates the constraint that the chip design be minimally affected and that the real estate occupied by the received level detector not be significantly increased. Thus, a straightforward extension of the techniques used in the customized integrated circuit chip prior art presents an undesirable expansion of the chip area occupied by the AGC circuits in the AFE chip. The present invention is an improvement in the art of stepped received level detectors, or AGC circuits, used in integrated circuit analog front ends for a modem which provides the desired expansion of dynamic range described above, without increasing the number of threshold level detectors in the previous design.